In order to improve the rigidity, dimensional stability, printability, etc. of polyolefines which are to be worked into various articles or which are to be foamed into foam plastic, attempts have been made to blend the polyolefine with some vinyl polymer, e.t. polystyrene. Due to the low compatibility of polystyrene in polyolefines, it has proved to be difficult to obtain homogeneous blends with an attractive appearance. For example, with regard to small amounts, 02-10% by weight, the blend gets a low impact resistance and an impaired appearance.
Attempts have been made to overcome the above problem by polymerizing styrene inside the polyolefine particles. Basically, two different methods of polymerizing styrene in situ in polyolefine particles have been suggested. Both methods are based on the fact that styrene is allowed to diffuse into the polyolefine particles, after which the polymerization of styrene occurs in a basically traditional way. The greatest difference between the two methods is the way in which styrene is allowed to diffuse into the polyolefine particles.
In West German patent DE 29 08 662, the polymerization of styrene in situ is described in several different polyolefines. In this method, styrene is added slowly into a water suspension containing polyolefine particles at an elevated temperature. Thus, the impregnation and polymerization of the styrene occur simultaneously, with the result that the polymerization tends to occur more in the surface regions of the polyolefine particles. This results in the polystyrene being concentrated on the surface layer of the polyolefine particles.
U.S. Pa. No. 4,412,938 describes the polymerization of styrene in situ in an ethylene vinyl acetate copolymer. In this method, all of the styrene is added at one time into a water suspension containing ethylene vinyl acetate copolymer particles. The styrene is added at so low a temperature that no polymerization occurs. Only after all styrene or most of the styrene has diffused into the ethylene vinyl acetate copolymer particles is the temperature increased and the polymerization started. A disadvantage of this method is that it lasts several hours before the styrene has diffused into the ethylene vinyl acetate copolymer particles. The method is, therefore, time-consuming.
Neste Oy's Finnish Patent Application No. 894461 (Corresponding to U.S. Pat. No. 5,164,456) describes a method for preparing a polyolefine-vinyl polymer composite in which method the impregnation occurs totally in the absence of water.